This invention relates generally to process control. In particular this invention relates to control of continuous processes using programmable controllers.
Continuous process control has traditionally been effected using dedicated analogue circuitry to measure process parameters and produce control signals. Analogue circuit elements were used to define control algorithms and were not assignable from one process to another. With the advent of high speed digital processors, analogue to digital converters, and digital to analogue converters, it has become possible to control continuous processes using general purpose programmable digital computing elements to execute control algorithms. Nevertheless, the response of digital systems to events occurring in rapidly changing circumstances is unsatisfactory in some circumstances for effecting the desired degree of process control. For example, in injection molding processes, large changes in pressure occur within milliseconds during the final stage of mold filling. To control clamping forces it is necessary to respond immediately to the occurrence of mold pressures marking a change in process activity.
Heretofore, it has been necessary to effect such immediate responses outside of the overall process control. Consequently, advantages of using a programmable control are lost with regard to those elements not directly controlled by the programmable control. Further, control circuitry associated with these elements is typically dedicated to particular control algorithms.